1. Field of the Invention
The invention relates to lumber sorting equipment, and more particularly to equipment for suspending slings on sling sorters.
2. Description of the Related Art
Like many businesses, the lumber industry has become highly mechanized. Machines convert forest timber to building lumber with minimal human assistance. The machines receive raw logs which are processed and cut to produce building lumber and a variety of other useful products. When this milling process is complete, the lumber must be sorted and packaged for delivery to users.
The modern sling sorter system, such as the Harvey Engineering and Manufacturing (HEMCO) inline planermill and green lumber sorting system, performs the lumber sorting operation rapidly and efficiently. A conveyor receives each board of lumber of assorted lengths. As the board is cut and trimmed to the particular specifications, a computer records the board's position on the conveyor.
The conveyor then moves the cut lumber along a set of parallel rails. The rails are divided into segments so that a set of segments may temporarily swing upward, creating an opening in the parallel rails. The segments are controlled by the computer. When a board of particular specifications reaches a point directly above a certain location on the rails, the rail segments are controlled by the computer. When a board of particular specifications reaches a point directly above a certain location on the rails, the rail segments automatically swing upward, and the board drops through the opening. The lumber is sorted by dropping all lumber of the same specification through the same opening, or additional bays, if required.
Beneath each opening are, typically, from two to five slings. The slings hang parallel to the rails across the opening, and are usually made of tough webbing. As the boards fall through the opening, they land on and are held by the slings.
One end of each sling is securely attached to a reel, or sling spool. The other end, which features a D-ring on which the sling webbing is permanently sewn, attaches to a hook which is permanently connected to the sorter structure. The reel, or sling spool, contains an amount of extra webbing. As the sling fills with lumber, the reel, or sling spool, is unwound so as to deliver more webbing, so that the sling remains essentially full without overloading. This feature minimizes damage to the boards by reducing the falling distance of the newly sorted lumber onto the lumber already being held in the sling.
The other end of each sling features a D-ring which attaches to a hook. When the sling is full and ready to be unloaded, the hook is withdrawn, thereby detaching the sling and allowing the sling to fall to the ground. After the sorted lumber is removed from the sling, the webbing is taken in again by the reel, or sling spool. The webbing passes through a metal retriever attached to the end of the reel, or sling spool. Once the reel, or sling spool, has pulled the sling webbing through the metal retriever until the D-ring is firmly fitted into the slot at the free end of the metal retriever, the retriever swivels upwards approximately 90 degrees so that it extends horizontally across the bay opening.
The hook is made to extend toward the reel, or sling spool, and catches the D-ring at the end of the webbing. The retriever and hook rotate back to their vertical hanging positions, with the sling hanging loosely between them. The sling is now ready to receive the next load of lumber.
Onto the end of the webbing which attaches to the hook is permanently sewn a metal D-ring. The D-ring, as the name indicates, is typically shaped like the letter "D" or is approximately triangular in shape. The end of the webbing loops over the base of the triangle and is securely sewn to itself. The hook catches the apex of the triangular D-ring to hold the sling and its lumber. When the hook is withdrawn, the D-ring falls to the ground. The webbing is reeled in until the D-ring contacts the metal retriever. After the retriever shifts to the horizontal position, the hook extends and lowers onto the D-ring as the sleeve swings back to the vertical position.
The D-ring does not always fall to the ground after being released by the hook. The D-ring often lands on the platform supporting the sorted lumber that has just been released by the sling. When this occurs, the D-ring must be pulled underneath the pile of lumber in the process of reeling in the webbing. The flat construction of the D-ring usually prevents significant damage to the lumber.
The webbing of the slings, especially the portion in the immediate vicinity of the D-ring, is subject to tremendous wear, and must periodically be replaced. The webbing usually wears out initially where it attaches to the D-ring. This is sometimes due to the sharp and rough edges of the D-ring which tear and weaken the webbing. The webbing is also damaged when the D-ring is dropped from the hook and hits the floor below. Many mills have concrete or other hard surface floors, and the webbing is often crushed between the floor and the falling D-ring. The webbing suffers further as it is dragged along the floor under the weight of the metal D-ring as the sling is reeled in to begin another load cycle. The webbing deteriorates even more rapidly when it is caught by and dragged under the pile of lumber.
The constant wear reduces the useful lifetime of the webbing. To prevent a failure, the webbing must be periodically replaced. This involves shutting down the entire sorter apparatus while the old sling is replaced. The mill must pay for the material and labor for the replacement, and the temporary loss of the entire sorter apparatus lowers the sorter system efficiency.